all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tka/key.go

@@ -7,6 +7,7 @@ import (
 	"crypto/ed25519"
 	"errors"
 	"fmt"
+	"maps"
 
 	"tailscale.com/types/tkatype"
 )
@@ -64,9 +65,7 @@ func (k Key) Clone() Key {
 
 	if k.Meta != nil {
 		out.Meta = make(map[string]string, len(k.Meta))
-		for k, v := range k.Meta {
-			out.Meta[k] = v
-		}
+		maps.Copy(out.Meta, k.Meta)
 	}
 
 	return out